Process of treating molybdenum ores and products resulting therefrom



Patented Jan. 16, 1940.

IWUNITED STATES.

PATENT OFFICE PROCESS OF TREATING MOLYBDENUM OREISVI AND PRODUCTS RESULTING THERE- FRO John D. Sullivan'and Dimitry-Niconofi, Columbus,

Ohio, assignors to Battelle Memorial Institute, 7 Columbus, Ohio, a corporationof Ohio i N o-Drawing.

Application September 3, 1938, Serial No. 228,449

6 Claims. (01. 23-24) u largestuse of molybdenum at the present time "is inmaking steel and cast iron alloys. For these purposes, the molybdenum is usually utilized either in the form of ferromolybdenum or calcium molybdate. In either case in order to meet specifications, for example, those of the American Society of Testing Materials, the copper and sulfur contents must be fairly low.

tone of the common prior art methods of making calcium molybdate is to" roast molybdenite concentrates to form molybdenum trioxide,

u salts, including calcium 'molybdate, by leaching.

For example, one patentee has suggested the roasting of molybdenite concentrates to form 4.0 molydenum trioxide, the leaching of the calcine withsodium hydroxide to form soluble sodium molybdate and the treating of the solution with lime to precipitate molybdenum as calcium molybdate. Another patentee has suggested the use of a somewhat "similar process but with the added. step of treating the sodium molybdate solution with a limited amount of a solution of a calcium salt to remove phosphates, silicates, and [the like before H precipitation ;,of the Mo as -59 CaMoO4. Still another patentee has suggested a processsimilar to the latter process, except that the purification of the sodium modybdate solution 1toremove phosphates, silicates, and the like is ;done by means of the reagent, calcium molyb- When MOS2 is roasted in air the reaction may be written as follows:

The M which is formed, however, has an ap- 5 preciable vapor pressure and in order to prevent undue] loss by volatilization, the temperature must be. kept fairly low, for example, approximately 500 to 600 C. At this temperature, however, it is extremely difiicult to eliminate completely the sulfur from sulfides of iron and copper which may be associated with the molybdenite. The problem, therefore, of obtaining calcium molybdate of a suitable commercial degree of purity from copper and iron-bearing molybdenite concentrates is different from that when the molybdenite concentrates are relatively pure.

One object of our invention is to produce molybdenum salts from copper-bearing molybdenite concentrates with a degree of purity satisfactory for commercial uses.

A further object of our invention is to produce calcium molybdate from copper-bearing molybdenite concentrates.

Afurther object is to produce calcium molybdate with a low sulfur content from ironand copper-bearing molybdenite concentrates.

A,further object of our invention is to effect removal of soluble sulfur from leach liquors 010- tained from leaching ironor copper-bearing molybdenum calcines.

Other objects will appear in the following description and the claims appended thereto.

The sequence of steps which we preferably utilize are as follows:

.1 Roasting molybdenite concentrates to convert substantially all the molybdenum to oxide.

. 2, Leaching the calcines with an alkali hydroxide or carbonate. I

3. Treatment of the leach liquor to precipitate soluble sulfates.

4. Separation of solution from residue.

I, 5. Precipitation of molybdenum from solution ,as CaMoOr. 6. Preparation of CaMoO4 in a form suitable as an article of commerce.

, 7. Return of regenerated solution to leach additional calcines.

Mol'ybdenite concentrates are first roasted by means well known to the art. The materials to be treated by our process may be high in copper andir'on sulfides although the process is not limited thereto, it being equally applicable to molybdenite relatively free from either iron or u copper sulfides. As has previously been pointed out, in order to prevent loss of molybdenum by volatilization, especially in roasting calcines containing iron and copper sulfides, we. ordinarily do not roast to complete elimination of sulfur. We have found that most of the sulfur remaining in the calcines is sulfate sulfur and is soluble in alkali hydroxide solutions.

solution of an alkali metal hydroxide such assodium hydroxide. We mayulea'ch at. ordinary room temperature or at elevated temperatures, However, since the rate of dissolution is fairly rapid we ordinarily prefer to leach at room temperature. The amount of sodium hydroxide used in the leach liquor is not critical but we prefer to have a slight excess present over that-theoretical.- ly required to meet the chemical requirements.

For example, at room temperature we leached traction of Mo was not changed materially.

We have discovered, however, that we may use sodium carbonate instead of sodium hydroxide as the leaching solvent. Inasmuch as carbonates should be absent in the subsequent step of our process involving precipitation of molybdenum as calcium molybdate, as otherwise some calcium carbonate will also precipitate and thus lower the molybdenum content of thecalcium molybdate, we have found it desirable to react thesodium carbonate in solution with an excess of calcines, preferably at boiling temperature. The M003 in solution forms molybdic acid which reacts with and decomposes the sodium carbonate, the carbon dioxide passing off as agas. In this specification where we speak of sodium hydroxide as the leaching solvent it will be understood that we can use sodium carbonate.

Also, as will be discussed at greater length later, we do not have to use fresh sodium hydroxide solution. In the preferred form of our invention we use a cyclic process and use" regenerated sodium hydroxide solution to leach the calcines.

When the leaching action is substantially finished we precipitate soluble sulfates in the solution as barium sulfate. We remove sulfates in this manner because it is a simpler operation than first giving the calcines a preliminary water leach to remove soluble sulfates. Also, ifawaterleach is used some molybdenum is likewise dissolved and must be recovered from the solution by some method. l We have found that a strong alkali solution of molybdenum can contain appreciable amounts of sulfate sulfur and that if a deficiency of lime is added to precipitate the molybdenum as much as 20 grams per liter of S as S04. can be tolerated in the solution and the calcium molybdate precipitated will be substantially free-of sulfur. Ina "cyclic process, however, the sulfate sulfur will build up to the saturation point and thereafter th calcium molybdate will be contaminated with sulfate. v

vWe have found that barium sulfate is more "insoluble than barium molybdate in an alkaline solution such as thatused in" leaching the cal- .cines in our process, or the solution immediately is substantially free of sulphur.

before filtering. We have discovered that if we add a solution of a soluble barium salt, for example barium chloride, to the solution in an amount theoretically required to precipitate all the sulfate, we can precipitate substantially all of the sulfate from the solution withoutlprecipitation of substantially any of the molybdenum as bariu1n molybdate. he roasted concentrates are leached with a By adding slightly less than enough barium chloride to precipitate all the sulfate we are enabled to remove sufiicient sulfate from the solution, with no removal or at the most with a minor removal of molybdenum, so that the calcium molybdate precipitated in a subsequent step As an example we took a solution containing 40 grams per liter Mo and 4 grams per liter free NaOH. We added barium chloride in amount sufficient theoretically to precipitate per cent of the sulfate present. The precipitate contained only 0.03 per cent of Mo- When barium chloride was added in amount sufficient theoretically to precipitate 100 per cent of the sulfate present, theprecip'ate contained 0.57 per cent of Mo. i

t is understood-of course,.that we use a solution of a soluble salt of barium for convenience. Removal of sulfates can be effected by adding solid barium salts, for example, chloride, oxide or hydroxide, or by adding an emulsion such as that of barium oxide or hydroxide. i

After the leaching operation and the addition of barium chloride to precipitate soluble sulfates, the pregnant solution is removed from the residue, for example, by filtering. We prefer that the wash solutions, at least the first, contain som free sodium hydroxide. Theresidue, after filtering; will contain an appreciable amount of pregnant liquor, and if the concentration of sodium hydroxide in this is high, enough we can wash with water, but if the concentration is low, we prefer to add about 1 per cent-of free sodium hydroxide to the first wash water.

The residue contains the gangue material present,,and also the iron, copper, and precipitated barium sulfate.v The pregnant solution contains any purification steps to remove these impurities.

We may, therefore, treat the sodium molybdate solution with lime or calcium hydroxide to precipitate the molybdenum as calcium molybdate. The precipitation of molybdenum as calcium molybdate isbest done, at temperatures approaching the boiling point of water. If we add anhydrous or quick lime we find that the solution is heated to a substantial amount, usually to about 60 C., by reaction of the lime with the solution :and so extraneous heat need be used only to increase the. temperature .to substantially. the

boiling point.

Ordinarily we prefer not to precipitate all of the molybdenum in'the' solution as calcium molybdate; so we usually add lime to the extent of about '75 or per cent of the theoretical amount required to precipitate all of the molybdenum present. The reason we" prefer not to 'precipitate-allthe molybdenum is that by using a deficiency of lime we are enabled to produce *a iez ies calcium molybdate of higher molybdenum content.

The calcium molybdate formed in this manner is crystalline and granular and can easily be separated by filtration or by decantation. Washing can be done with Water. Inasmuch as the time involved in the precipitation step may be of the order of two hours or longer, and since the solutions are maintained at nearly the boiling point, considerable water is evaporated if open vessels are used. We have found, in fact, that we can control the process so that the amount of water evaporated in this step will compensate for that added to the circuit in the various washing steps involved.

The calcium molybdate formed in this manner is dried and is ready for marketing; It contains 44 to 46 per cent of molybdenum whereas pure calcium molybdate contains only 48 per cent.

We may, for example, put it in cloth or paper We preferably briquette the calcium molybdate in large units such as brick or blocks, each unit containing a definite weight of molybdenum. This product can be fed into a metallurgical furnace, and the operator knows the amount of molybdenum added in eachunit. This is a convenient, and We believe novel, form of calcium molybdate for metallurgical use.

When the molybdenum is precipitated as calcium molybdate the reaction may be written as follows:

The sodium hydroxide, therefore, is regenerated. The regenerated solution containing sodium hydroxide and sodium molybdate is re-used as a leaching solvent for more calcines. In other words, in its preferred form our process is cyclic and the only leaching solute necessary to add is an amount sufficient to replace the small amount of sodium hydroxide which may be lost in the process. As previously stated, we may use sodium carbonate for the make-up alkali. If so, we prefer to make an aqueous solution of the carbonate either in water or regenerated solution and react it at a temperature approaching boiling with an excess of calcines.

Briefly summarized, therefore, our process of making substantially pure calcium molybdate comprises roasting molybdenite ores or concentrates to form molybdenum trioxide. The concentrates may contain appreciable quantities of iron or copper sulfides because in our process the effect of either. the copper or the iron, or the sulfur associated with them is rendered harmless. In our cyclic process the calcines are leached with a solution of sodium hydroxide containing sodium molybdate. We add to this solution any new sodium hydroxide that may be required. After or during the leaching process we add a solution of a soluble barium salt or its equivalent to precipitate substantially all the soluble sulfates as barium sulfate. We then filter and wash to obtain a filtrate of substantially pure sodium molybdate solution and a residue containing gangue, barium sulfate, and iron and copper oxides or hydroxides. The sodium molybdate solution is treated at a temperature approaching boiling with about '75 to 80 per cent of the theoretical amount of lime required to precipitate all the molybdenum as calcium'molybdate. After this reaction is completed, We filter and wash the calcium molybdate '5 and return the regenerated sodium hydroxide solution to the head of the leaching cycle. The calcium molybdate is dried and bagged or put into a suitable formfor subsequent use. It is obvious, of course, that our process can be-m worked on a batch basis and that the cyclic procedure while desirable from an economic standpoint is not essential to our invention. It is possible that a strontium salt may be utilized instead of a barium salt to precipitate soluble sulfate.

As an alternative method of removing sulfates from the solution, we can perform this step after precipitation of the calcium molybdate. As has previously been pointed out, a strong alkali solu-" tionmay contain in excess of 20 grams per liter of sulfur as sulfate and if a deficiency of lime is added to precipitate part of the molybdenum as calcium molybdate, sulfates are not precipi tated. As we add only '75 to 80 per cent of the li'i theoretical amount of lime required to precipitate all the molybdenum, sulfates will not co-precipitate with the molybdate unless the concentration reaches a fairly high amount. We may, therefore, after addition of lime to precipitateiiio the molybdenum as molybdate and separation of the precipitate from the residue, add a soluble salt of barium to precipitate the sulfate as barium sulfate. If the solution is hot, the reaction is quite rapid. We then separate the barium sulfate from the solution, for example, by filtering, and the solution can be used to leach calcines. We can add the theoretical amount of barium salt to precipitate all the sulfate or somewhat less than the theoretical amount so that' the precipitated barium sulfate will be substantlally free of molybdenum. Since a considerable amount of sulfate can be tolerated in the solution without danger of contamination of the calcium molybdate with sulfate it is only moss-1.45 sary that we remove sulfate from solution so that this critical amount is not exceeded.

Thus, for example, we have taken a solution containing 37.8 grams per liter molybdenum,

42.8 grams per liter free sodium hydroxide andpgo 12.5 grams per liter sulfur as sulfate, and added slightly more than the theoretical amount of barium chloride to precipitate all the sulfate. The resulting solution contained only 0.4 gram per liter sulfur.

If the concentrates contain appreciable amounts of arsenic, phosphorus and the like, which are rendered soluble in the process, it may be necessary to remove them from solution, prior to precipitation of the molybdenum as calcium o molybdate. The treatment which we give the solution with a barium salt to remove sulfates also effects removal of phosphates, arsenates and the like, and in most cases, a subsequent treatment for their removal is unnecessary. In cases where 6 it is desirable to effect virtually complete removal of these impurities, for example, in the manufacture of molybdenum chemicals, we may treat the solution by one of the methods well known to the art. For example, we may roast the molybdenite concentrates to form molybdenum trioxide, leach the calcines with sodium hydroxide to form soluble sodium molybdate, precipitate the excess sulfate as barium sulfate, treat the purified sodium molybdate solution with a remaining phosphates, silicates and the like and then treat the solution with lime to precipitate the molybdenum as calcium molybdate.

It is obvious, of course, that the purification step which we employ in removing sulfates from the solution to give a relatively pure solution of sodium molybdate is applicable in processes of making other molybdenum salts. For example, the sodium molybdate solution purified from soluble sulfates by our process need not be used to form an alkaline earth molybdate but by suitable treatment can be converted into other molybdenum compounds.

We may use potassium or other alkali hydroxides instead of sodium hydroxide as a leaching agent. We prefer, however, to use sodium hydroxide because it is cheaper. Likewise, we may use other alkaline earth oxides, for example, those of barium, strontium or magnesium instead of lime to precipitate the respective molybdates. It is obvious that we may precipitate the alkaline earth molybdates with. soluble salts of alkaline earth metals, but we prefer to use the oxides or hydroxides because in this manner the alkali hydroxide is regenerated.

We have perfected a simple and inexpensive process for the removal of soluble sulfates from molybdenum solutions. By following out our process it is possible to make calcium molybdate of sufficient purity to meet usual specifications from very impure ores or concentrates containing 7 appreciable amounts of copper and iron sulfides and we have thus been able to extend the uses of these impure concentrates and ores to applications Wherein they have hitherto been unsuitable. We have also invented a novel process for the removal of soluble sulfates from molybdenum solutions, and have developed a process that is cheaper than any heretofore used or proposed. It will be obvious, therefore, that we have made an important development in the art.

Having thus described our invention, what we claim is: r

1. In a process of recovering molybdenum substantially free from sulfate from materials containing molybdenum and sulfate sulfur, the combination of steps comprising leaching said material with a solution of an alkali metal compound to form an alkaline solution of an alkali limited amount of calcium salt to remove .the

metal molybdate and sulfate, adding to the said molybdate solution a compound of an element from the group consisting of barium and strontium, said compound being substantially free from molybdenum and being added in an amount insufficient to precipitate all of the sulfate present but sufficient to reduce the sulfate sulfur content of the solution below 20 grams per liter substantially without precipitating molybdenum, separating the solution from the precipitated sulfate, and precipitating upto per cent of the molybdenum content of the solution substantially free from sulfate.

2. In the process of claim 1, adding barium hydroxide to the molybdate solution to precipitate barium sulfate.

3. The process of claim 1, wherein the sulfate is precipitated from the molybdate solution in the presence of residue remaining after leachmg.

4. The process of claim 1, wherein molybdenum is precipitated from the solution as calcium molybdate.

5. The process of claim 1, wherein the solution remaining after precipitating the molybdenum is returned to the leaching stage and con tacted with a further quantity of said material to remove molybdenum therefrom.

6. 'In a cyclic process of making calcium molybdate by leaching materials containing molybdenum and sulfate with a solution of sodium hydroxide and precipitating calcium molybdate from said solution, the combination of. steps comprising maintaining the sulfate sulfur concentration of said solution below 20 grams per liter at the molybdate precipitating stage, by precipitating sulfate from the recirculated solution with a compound of an element from the group consisting of barium and strontium substantially without precipitating molybdenum therefrom, said compound being substantially free from molybdenum and being employed in an amount insufiicient to precipitate all of the sulfate from the solution, and precipitating up to 90 per cent of the molybdenum from the solution as calcium molybdate substantially free from sulfate.

JOHN D. SULLIVAN. DIMITRY NICONOFF. 

